BEIJING, April 20 (Reuters) - China has expressed concern over the "forced interception" by the U.S. of an Iranian-flagged cargo ship, a Chinese foreign ministry spokesperson said on Monday, urging relevant parties to abide by the ceasefire agreement in a responsible manner.

"The situation in the Strait of Hormuz is sensitive and complicated," said spokesman Guo Jiakun during a regular press briefing. Parties involved should avoid further escalation and "create the necessary conditions for normal transit through the strait to resume," he added.

The U.S. said earlier it fired on and seized an Iranian cargo ship that tried to ‌run its blockade of Iranian ports. Iran's military said the ship had been travelling from China and vowed retaliation against what it called "armed piracy by the U.S. military."

Beijing on Monday also urged relevant parties to "continue to maintain the momentum of the ceasefire and negotiations".

"Now that a window for peace has opened, favourable conditions should be created to bring the war to an end as soon as possible," Guo said.

(Reporting by Liz Lee; Writing by Xiuhao Chen; Editing by Christian Schmollinger and Raju Gopalakrishnan)